Field of the Invention
Most stethoscopes used today have a dual chest-piece type of head with a switch valve for selectively connecting the sound tube either to a low frequency bell or to a high frequency diaphragm. Both the bell and the diaphragm are necessary when a person skilled in heart problems desires to listen very closely to the heart. For example, the diaphragm is useful in listening to sounds emanating from a heart in the range of 100 to 2,000 Hz. The diaphragm may be rather large, since it is not necessary that is be sealed to the skin of the patient; it is only necessary that it touch the patient's skin on at least one portion of the diaphragm surface.
On the other hand, the bell must be sealed about its periphery to a patient's skin since the skin itself is used as a diaphragm. Consequently, the bell is useful in the low frequency range of 50 to 150 Hz. It has been found that the deeper the bell is, the better response it actually provides to the listener within this frequency range.
In prior art stethoscopes, in order to have a deep bell, it was necessary that the bell axis be offset from the diaphragm axis 90.degree.. A valve was situated on the intersection of the axis of the bell and the diaphragm. Thus, either the bell or the diaphragm could be shut off when not in use to eliminate spurious sounds. While this arrangement produced an excellent stethoscope with high acoustical quality, it resulted in a large mass in the head, making the total stethoscope very heavy and, consequently, difficult both to wear and to carry. Further, such stethoscopes required that the doctor or listener use two hands when switching from one microphone mode to the other. In other words, if the listener were employing the diaphragm and wished to employ the bell, he would have to use his free hand to operate the switch. If the patient required support, the presence of a nurse or other attendant was therefore required. Further, this stethoscope suffered from the deficiency of being rather difficult to employ in both modes in a confined space, such as under a shirt, or within a brassiere. Consequently, it was almost always necessary for the patient to at least partially disrobe in order for the doctor to listen to the patient's heart in both of the head modes.
In the more recent past, it has become somewhat common to provide a dual head in which the bell and the diaphragm are coaxially aligned on the opposite sides of the head body. An example of such a stethoscope has been shown in my prior U.S. Pat. No. 3,472,335. In that patent, a movable switch was positioned near the axis of the head to allow the listener to use one hand when switching from one microphone mode to the other. However, in order to be totally practical, that head resulted in a compromise which has proven to be somewhat unacceptable. To prevent the thickness of the head from becoming so cumbersome as to be impractical, it was necessary to reduce the depth of the bell. As a result, the stethoscope is unable to detect and transmit sounds generated by the heart in the lowest desirable frequencies.
Consequently, it has become necessary to provide a dual head having a bell with the maximum possible depth. It has been preferred that such a head have a relatively low mass to provide comfort for the wearer and ease for the person carrying the stethoscope. It is also preferred that such a stethoscope have a minimum overall height in order to make the stethoscope easier to use with a patient who has remained clothed, etc.